Throughout the world, considerable oil reserves are locked in the form of tar sands, also called bitumen sands. The hot water extraction process is the standard process for recovering bitumen from the sand and other material in which it is bound. The bitumen is then treated to obtain a synthetic crude oil therefrom.
In the hot water extraction process using existing extraction facilities, tar sand is first conditioned in large conditioning drums or tumblers with the addition of caustic soda (sodium hydroxide) and hot water at a temperature of about 180.degree. F. The nature of these tumblers is well known in the art. The tumblers have means for steam injection and further have retarders, lifters and advancers which create violently turbulent flow and positive physical action to break up the tar sand and mix the resultant mixture vigorously to condition the tar sands. This causes the bitumen to be aerated and separated to form a froth.
The mixture from the tumblers is screened to separate the larger debris and is passed to a separating cell where settling time is provided to allow the aerated slurry to separate. As the mixture settles, the bitumen froth rises to the surface and the sand particles and sediments fall to the bottom to form a sediment layer. A middle viscous sludge layer, termed middlings, contains dispersed clay particles and some trapped bitumen which is not able to rise due to the viscosity of the sludge. The froth is skimmed off for froth treatment and the sediment layer is passed to a tailings pond. The middlings is often fed to a second stage of froth floatation for further bitumen froth recovery. The water/clay residue from this second stage is combined with the sediment layer from the separating cell for disposal in the tailing ponds.
Recently, a modified hot water extraction process termed the hydrotransport system has been tested. In this system, the tar sand is mixed with hot water and caustic at the mine site and the resultant mixture is transported to the extraction unit in a large pipe. During the hydrotransport, the tar sand is violently mixed and aerated by turbulent flow and by injection of air at intermittent points along the pipe. As a result, the tar sand is conditioned and the bitumen is aerated to form a froth. This system replaces the manual or mechanical transport of the tar sands to the extraction unit and eliminates the need for tumblers.
The bitumen froth from either process contains bitumen, air, solids and trapped water. The solids which are present in the froth are in the form of clays, silt and some sand. From the separating cell the froth is passed to a defrother vessel where the froth is heated and broken to remove the air. Naphtha is then added to cause a reduction in the density of the bitumen, facilitating separation of the water and solids from the bitumen by means of a subsequent centrifuge treatment. The centrifuge treatment first includes a gross centrifuge separation followed by high speed centrifuge separations. The bitumen collected from the centrifuge treatment usually contains less than 2% water and solids and can be passed to the refinery for upgrading. The water and solids released during the centrifuge treatment are passed to the tailings pond.
The tailings in the tailing pond are largely a sludge of caustic soda, solids and water with some bitumen. During the initial years of residence time, some settling takes place in the upper layer of the pond, releasing some of the trapped water. The water released from the sludge can be recycled back into the hot water process. The major portion of the tailings remains as sludge indefinitely. The sludge contains some bitumen and high percentages of solids, mainly in the form of suspended silt and clay.
The tailings ponds are costly to build and maintain. The size of the ponds and their characteristic caustic condition creates serious environmental problems. In addition, environmental concerns exist over the large quantity of water which is required for extraction and which remains locked in the tailings pond after use.
It is known that sludge is formed in the initial conditioning of the tar sand, when the caustic soda attacks the silt and clay particles. The caustic soda causes the clays to swell and disburse into platelets. These platelets are held in suspension and form the gel-like sludge. Expanding-type clays such as the montmorillanite clays are particularly susceptible to caustic attack. Because of the problems caused by sludge formation and the low bitumen recovery available from highly viscous sludges, lower grade tar sands containing high levels of clays cannot be treated satisfactorily using the hot water extraction process.
The need exists for an extraction process which would result in a reduction or elimination of the production of sludge and therefore an increase in the water available for recycling. Any such process would also provide the possibility of increased bitumen recovery from medium and lower grade ores.
Also it is desirable that any tar sand extraction process should maintain or increase the present throughput possible by use of existing extraction processes and thereby not increase the cost of extraction. It is further desirable that a tar sand extraction process be of use in conventional extraction facilities. It is also desirable to eliminate the hazardous caustic used in today's commercial units.
Alternate processes, such as that described in U.S. Pat. No. 4,120,777, have been proposed which include the use of alternate conditioning agents such as soluble metal bicarbonates. However, such processes have generally not been adopted by the industry for a number of reasons. For example, proposed processes often increase the cost of extraction beyond reasonable levels by requiring the use of large amounts of agents or by reducing the rate at which tar sand can be processed. In addition, such processes are not readily adopted since they cannot be carried out in existing extraction facilities.